In the case of papermaking machines according to the invention, a generally inward water extraction is attained as on a horizontal screen section of the inner screen upstream of the subsequent dewatering roller means while, as both inner and outer screens pass over the dewatering roller means a generally outward water extraction, in a direction functionally opposite to the inward water extraction, is attained. Consequently, a paper thusly produced has approximately the same paper properties on both sides.
The fibre suspension obtained through the passage of the material onto the inner screen already has most of the water extracted on the horizontal screen section of the inner screen which action is normally intensified as by suction boxes placed beneath such horizontal screen section. However, as to subsequent additional extraction of water, the prior art, heretofore, has considered it sufficient if the paper web was merely subjected to a change of direction. For example, in the prior art, it was only thought possible to attain a more substantial water extraction through measures of positive or negative pressurization as, for instance, through suction boxes beneath the horizontal inner screen section as in the Federal Republic of Germany patent specification No. 2,411,739 or through water-removal scrapers positioned after the roller as in Federal Republic of Germany Published patent specification No. 2,226,029. This teaching and belief by the prior art applies to both high-speed and slow-running papermaking machines; that is, in the case of high-speed machines a great deal of water is already flung out with a small turning of the paper web as a result of the high centrifugal forces, while with slow-running machines only a little water is expelled and, in the opinion of the experts in the prior art, in both cases only longer suction boxes or a greater vacuum in the horizontal screen section of the inner screen were worthy of consideration for an intensification of the water extraction.
In the case of the papermaking machine taught by and disclosed in the said German patent specification No. 2,411,739 the roller has a perforated shell from which the two screens run off together still within the enveloping upper roller quadrants.
After running off the roller water appears on both the inner and the outer screens and such water has to be removed by a plurality of scrapers. The outer screen exhibits a particularly large quantity of water as a result of the water being driven outwards of the roller through the screen tension and centrifugal force. A part of such water is indeed flung off; however, the remainder of the water is collected on and carried by the outer screen. In such prior art arrangements, water is also flung back onto the inner screen from the perforations of the roller shell. The comparatively large quantity of residual water on the screens downstream of the roller results in an undesirable re-moistening of the paper web. Further, the scrapers normally inflict some wear on the screens and, given the slightest irregularity in either their shape or in the screens, severe damage to these sensitive screens occurs.
In the case of the papermaking machine taught in the said German patent specification No. 2,226,029 the roller shell is likewise perforated. Through this a large part of the water extracted from the paper web is held in the inner screen and such water is subsequently removed from the inner screen by scrapers that are disposed after the roller.
It is the object of the invention to solve the prior art problem of attaining a greater extraction of water without the incorporation of costly devices and without enlarging the bulk of the machine, and this even when the machine is operated with a relatively slow web speed, and wherein the rollers can be inexpensive to produce and the problems associated with the prior art scrapers can to be avoided.